Photonic mems and structures

ABSTRACT

An interference modulator (Imod) incorporates anti-reflection coatings and/or micro-fabricated supplemental lighting sources. An efficient drive scheme is provided for matrix addressed arrays of IMods or other micromechanical devices. An improved color scheme provides greater flexibility. Electronic hardware can be field reconfigured to accommodate different display formats and/or application functions. An IMod&#39;s electromechanical behavior can be decoupled from its optical behavior. An improved actuation means is provided, some one of which may be hidden from view. An IMod or IMod array is fabricated and used in conjunction with a MEMS switch or switch array. An IMod can be used for optical switching and modulation. Some IMods incorporate 2-D and 3-D photonic structures. A variety of applications for the modulation of light are discussed. A MEMS manufacturing and packaging approach is provided based on a continuous web fed process. IMods can be used as test structures for the evaluation of residual stress in deposited materials.

This application is a divisional patent application of U.S. patentapplication Ser. No. 09/413,222, now U.S. Pat. No. 6,171,162, filed onOct. 5, 1999, now allowed, which is a continuation-in-part of U.S.patent application Ser. No. 08/744,253, now U.S. Pat. No. 5,986,796,filed Nov. 5, 1996, which is a continuation-in-part of Internationalpatent application Serial No. PCT/US95/05358, filed May 1, 1995, whichis a continuation-in-part of U.S. patent application Ser. No.08/238,750, filed May 5, 1994, now issued as U.S. Pat. No. 5,835,255.

BACKGROUND

This invention relates to interferometric modulation.

Interferometric modulators (IMods) modulate incident light by themanipulation of the optical properties of a micromechanical device. Thisis accomplished by altering the device's interferometric characteristicsusing a variety of techniques. IMods lend themselves to a number ofapplications ranging from flat panels displays and optical computing tofiber-optic modulators and projection displays. The differentapplications can be addressed using different IMod designs.

SUMMARY

In general, in one aspect, the invention features an IMod based displaythat incorporates anti-reflection coatings and/or micro-fabricatedsupplemental lighting sources.

In general, in one aspect, the invention features an efficient drivescheme for matrix addressed arrays of IMods or other micromechanicaldevices.

In general, in one aspect, the invention features a color scheme thatprovides a greater flexibility.

In general, in one aspect, the invention features electronic hardwarethat can be field reconfigured to accommodate different display formatsand/or application functions.

In general, in one aspect, the invention features an IMod design thatdecouples the IMod's electromechanical behavior from the IMod's opticalbehavior.

In general, in one aspect, the invention features an IMod design withalternative actuation means, some one of which may be hidden from view.

In general, in one aspect, the invention features an IMod or IMod arraythat is fabricated and used in conjunction with a MEMS switch or switcharray, and/or MEMS based logic.

In general, in one aspect, the invention features an IMod that can beused for optical switching and modulation.

In general, in one aspect, the invention features IMods that incorporate2-D and 3-D photonic structures.

In general, in one aspect, the invention features a variety ofapplications for the modulation of light.

In general, in one aspect, the invention features a MEMS manufacturingand packaging approach based on a continuous web fed process.

In general, in one aspect, the invention features IMods used as teststructures for the evaluation of residual stress in deposited films.

DESCRIPTION

FIG. 1A is a cross-section of a display substrate incorporating ananti-reflection coating and integrated supplemental lighting. FIG. 1Breveals another scheme for supplemental lighting.

FIG. 2 shows detail of the fabrication process of a micromachined arclamp source.

FIG. 3 illustrates a bias centered driving scheme for arrays of IMods ina display.

FIG. 4A is a diagram which illustrates a color display scheme based onthe concept of “base+pigment”. FIG. 4B reveals a block diagram of asystem that provides for field reconfigurable display centric products.FIG. 4C illustrates the concept as applied to a general-purposedisplay-centric product.

FIG. 5A is a diagram revealing an IMod geometry that decouples theoptical behavior from the electromechanical behavior, shown in theun-actuated state. FIG. 5B shows the same IMod in the actuated state.FIG. 5C is a plot showing the performance of this IMod design in theblack and white state. FIG. 5D is a plot showing the performance ofseveral color states.

FIG. 6A shows a diagram of an IMod that similarly decouples the opticalbehavior from the electromechanical, however the support structure ishidden. FIG. 6B shows the same design in the actuated state.

FIG. 7A illustrates an IMod design that utilizes anisotropicallystressed membranes, in one state. FIG. 7B shows the same IMod in anotherstate.

FIG. 8A is an illustration showing an IMod that relies on rotationalactuation. FIG. 8B reveals the fabrication sequence of the rotationalIMod design.

FIG. 9A is a block diagram of a MEMS switch. FIG. 9B is a block diagramof a row driver based on MEMS switches. FIG. 9C is a block diagram of acolumn driver based on MEMS switches. FIG. 9D is a block diagram of aNAND gate based on MEMS switches. FIG. 9E is a block diagram of adisplay system incorporating MEMS based logic and driver components.

FIG. 10A is a drawing that reveals the structure, fabrication, andoperation of a MEMS switch. FIG. 10B illustrates two alternative switchdesigns.

FIG. 11A is a drawing that shows examples of microring based 2-Dphotonic structure. FIG. 11B is a drawing of a periodic 2-D photonicstructure.

FIG. 12 is a diagram which revealing an example of a 3-D photonicstructure.

FIG. 13A is a drawing illustrating an IMod incorporating a microringstructure in the un-actuated state. FIG. 13B is the same IMod in theactuated state. FIG. 13C shows an IMod incorporating periodic 2-Dphotonic structure.

FIG. 14A illustrates and IMod design which acts as an optical switch.FIG. 14B shows a variation of this design that acts as an opticalattenuator.

FIG. 15A is a diagram of an IMod design that functions as an opticalswitch or optical decoupler. FIG. 15B illustrates how combinations ofthese IMods can act as a N×N optical switch.

FIG. 16 shows the fabrication sequence for a tunable IMod structure.

FIG. 17A illustrates how the tunable IMod structure can be incorporatedinto a wavelength selective switch. FIG. 17B further illustrates how thewavelength selective switch may incorporate solid state devices. FIG.17C illustrates how bump-bonded components may be intergrated.

FIG. 18A is a schematic representation of a two-channel equalizer/mixer.FIG. 18B illustrates how the equalizer/mixer may be implemented usingIMod based components.

FIG. 19 is a diagram illustrating a continuous web-based fabricationprocess.

FIG. 20 illustrates how IMod based test structures may be used as toolsfor stress measurement.

FIGS. 21A-21C Describe.

Anti-reflective Coatings

An attribute of one previously described IMod design (the inducedabsorber design described in U.S. patent application Ser. No.08/554,630, filed on Nov. 6, 1995, and incorporated by reference) is theefficiency of its dark state, in which it can absorb as much as 99.7% oflight which is incident upon it. This is useful in reflective displays.In the described design, the IMod reflects light of a certain color inthe un-actuated state, and absorbs light in the actuated state.

Because the IMod array resides on a substrate, the potential forabsorption is diminished by the inherent reflection of the substrate. Inthe case of a glass substrate, the amount of reflection is generallyabout 4% across the visible spectrum. Thus, despite the absorptivecapability of the IMod structure, a dark state can only be as dark asthe front surface reflection from the substrate will permit.

One way to improve the overall performance of an IMod based display isby the incorporation of anti-reflection coatings (AR coatings). Thesecoatings can comprise one or more layers of dielectric films depositedon the surface of a substrate and are designed to reduce the reflectionfrom that surface. There are many different possible configurations forsuch films and design and fabrication is a well known art. One simplefilm design is a single coating of magnesium fluoride approximatelyone-quarter wave thick. Another example utilizes a quarter wave film oflead fluoride deposited on the glass, followed by a quarter wave film ofmagnesium fluoride, with yet a third example interposing a film of zincsulfide between the two.

FIG. 1A illustrates one way in which an AR coating may be incorporatedinto an IMod display to improve the performance of the display system.In FIG. 1A, AR coating 100, which, as stated, could comprise one or morethin films, is deposited on the surface of glass layer 102 bonded toglass substrate 106, on the opposite side of which is fabricated IModarray 108. The presence of AR coating 100 reduces the amount of incidentlight 109 reflected from the surface by coupling more of it into theglass layer 102. The result is that more of the incident light is actedupon by the IMod array and a darker display state can be obtained whenthe IMod is operating in the absorptive mode. The AR coating 100 couldalso be deposited directly on the surface of glass substrate 106 on theside opposite that of the IMod array.

Integrated Lighting

FIG. 1A also shows how supplemental lighting may be supplied to such adisplay. In this case an array of microscopic arc lamps, 104, isfabricated into glass layer 102. Arc lamps are efficient suppliers oflight. Historically, arc lamps have been fabricated using techniquesrelevant to the fabrication of ordinary light bulbs. A typical versionof such a lamp is described in U.S. Pat. No. 4,987,496. A glass vesselis built, and electrodes, fabricated separately, are enclosed in thevessel. After filling with an appropriate gas, the vessel is sealed.Although such bulbs may be made small, their method of manufacture maynot be suited to the fabrication of large monolithic arrays of suchbulbs.

Techniques used in the manufacture of micromechanical structures may beapplied to the fabrication of microscopic discharge or arc lamps.Because of the microscopic size of these “micro-lamps”, the voltages andcurrents to drive them are significantly lower than those required tosupply arc lamps fabricated using conventional means and sizes. In theexample of FIG. 1A, the array is fabricated such that light 113 emittedby the lamps is directed towards the IMod array 108 by an inherentreflector layer 111, which is described below.

FIG. 2 provides detail on how one such lamp, optimized for a flat paneldisplay, could be fabricated. The sequence is described as follows. Asseen in step 1, glass layer 200 is etched to form a reflector bowl 201using wet or dry chemical etching. The depth and shape of the bowl aredetermined by the required area of illumination for each lamp. A shallowbowl would produce a broad reflected beam spread while a parabola wouldtend to collimate the reflected light. The diameter of the bowl couldvary from 10 to several hundred microns. This dimension is determined bythe amount of display area that can be acceptably obscured from theviewer's perspective. It is also a function of the density of the arrayof micro-lamps. Using standard deposition techniques, e.g., sputtering,and standard photolithographic techniques, a reflector/metal halidelayer 204 and sacrificial layer 202 are deposited and patterned. Thereflector/metal halide layer could be a film stack comprising aluminum(the reflector) and metal halides such as thallium iodide, potassiumiodide, and indium iodide. The metal halide, while not essential, canenhance the properties of the light that is generated. The sacrificiallayer could be a layer such as silicon, for example.

Next, electrode layer 206 is deposited and patterned to form twoseparate electrodes. This material could be a refractory metal liketungsten and would have a thickness that is sufficient to providemechanical support, on the order of several thousand angstroms. Thensacrificial layer 202 is removed using a dry release technique. Theassembly (in the form of an array of such lamps) is sealed by bonding toa glass plate like substrate 106 (shown in FIG. 1A) such that thereflector faces the plate. A gas, such as xenon, is used to backfill thecavities, formed by the lamps during the sealing process, to a pressureof approximately one atmosphere. This could be accomplished byperforming the sealing process in an airtight chamber that has beenpreviously filled with Xenon.

The application of sufficient voltage to the electrodes of each lampwill result in an electrical discharge, in the gas between the ends ofthe electrodes, and the emission of light 205 in a direction away fromthe reflector 204. This voltage could be as low as several tens of voltsif the gap spacing is on the order of several hundred microns or less.If the electrode material is deposited with minimal stress, thesacrificial layer, 202, will determine the position of the electrodeswithin the bowl. In this case, the thickness is chosen to position thedischarge at the focal point of the bowl. Should there be residualstress, which would cause the electrodes to move when released, thenthickness is chosen to compensate for this movement. In general thethickness will be some fraction of the depth of the bowl, from severalto tens of microns.

Referring again to FIG. 1A, the light is shown traveling along a path113. Thus light is emitted towards the IMod array, where it is acted onand subsequently reflected by the array along paths 110, towardsinterface 107 and the viewer 111.

The lamps may be fabricated without including the reflector layer sothat they may emit light omnidirectionally.

Lamps fabricated with or without the reflector may be used in a varietyof applications requiring microscopic light sources or light sourcearrays. These could include projection displays, backlights for emissiveflat panel displays, or ordinary light sources for internal (homes,buildings) or external (automobiles, flashlights) use.

Referring to FIG. 1B, an alternative supplemental lighting approach isshown. Light guide 118 comprises a glass or plastic layer that has beenbonded to substrate 112. Light source 116 which could comprise anynumber of emissive sources such as fluorescent tubes, LED arrays, or theaforementioned micro-lamp arrays, is mounted on opposite sides of thelight guide. The light 122 is coupled into the light guide using acollimator 120 such that most of the light is trapped within the guidevia total internal reflection. Scatter pad 124 is an area of the lightguide that has been roughened using wet or dry chemical means. Thescatter pad is coated with a material or thin film stack 126 whichpresents a reflective surface towards substrate 112 and an absorbingsurface towards the viewer 128.

When light trapped within the guide is incident upon the scatter pad,the conditions for total internal reflection are violated and someportion 129 of the light scatters in all directions. Scattered lightwhich would normally escape into the surrounding medium, i.e. towardsthe viewer, 128, is reflected into substrate 112 due to the presence ofthe reflective side of coating 126. Like the aforementioned micro-lamps,the scatter pads are fabricated in an array, with each pad dimensionedsuch that the portion of the display that it obscures from direct viewis hardly noticeable. While these dimensions are small, on the order oftens of microns, they can provide sufficient supplemental lightingbecause of the inherent optical efficiency of the underlying IMod array114. The shape of the scatter pad may be circular, rectangular, or ofarbitrary shapes which may minimize their perception by the viewer.

Addressing Elements In An Array

In order to actuate arrays of IMods in a coordinated fashion for displaypurposes, a sequence of voltages is applied to the rows and columns ofthe array in what is generally known as a “line at a time” fashion. Thebasic concept is to apply a sufficient voltage to a particular row suchthat voltages applied to selected columns cause corresponding elementson the selected row to actuate or release depending on the columnvoltage. The thresholds and applied voltages must be such that only theelements on the selected row are affected by the application of thecolumn voltages. An entire array can be addressed over a period of timeby sequentially selecting the set of rows comprising the display.

One simple way of accomplishing this is shown in FIG. 3. Hysteresiscurve 300 is an idealized representation of the electroptical responseof a reflective IMod. The x-axis shows applied voltage, and the y-axisshows amplitude of reflected light. The IMod exhibits hysteresisbecause, as the voltage is increased past the pull-in threshold, theIMod structure actuates and becomes highly absorbing. When the appliedvoltage is decreased, the applied voltage must be brought below therelease threshold in order for the structure to move back into theun-actuated state. The difference between the pull-in and releasethresholds produces the hysteresis window. The hysteresis effect, aswell as an alternative addressing scheme, is discussed in U.S. patentapplication Ser. No. 08,744,253, now U.S. Pat. No. 5,986,796, filed onNov. 5, 1996, and incorporated by reference. The hysteresis window canbe exploited by maintaining a bias voltage Vbias, at all times, to keepthe IMod in whatever state it was driven or released into. Voltages Voffand Von correspond to voltages required to actuate or release the IModstructure. The array is driven by applying voltages to the columns androws using electronics known as column and row drivers. IMods have beenfabricated with a pull-in threshold of 6 volts, and a release thresholdof 3 volts. For such a device, typical values for Vbias, Voff, and Vonare 4.5 volts, 0 volts, and 9 volts respectively.

In FIG. 3, timing diagram 302 illustrates the kind of waveforms that maybe applied to actuate an array of IMods that exhibit a hysteresis curveresembling curve 300. A total of five voltages, two column voltages andthree row voltages, are required. The voltages are selected such thatVco11 is exactly twice the value of Vbias, and Vco10 is zero volts. Therow voltages are selected so that the difference between Vsel F0 andVco10 equals Von, and the difference between Vsel F0 and Vco11 equalsVoff. Conversely, the difference between Vsel F1 and Vco11 equals Von,and the difference between Vsel F1 and Vco10 equals Voff.

The addressing occurs in alternating frames 0 and 1. In a typicaladdressing sequence, data for row 0 is loaded into the column driversduring frame 0 resulting in either a voltage level of Vco11 or Vco10being applied depending on whether the data is a binary one or zerorespectively. When the data has settled, row driver 0 applies a selectpulse with the value of Vsel F0. This results in any IMods on columnswith Vco10 present becoming actuated, and IMods on columns with Vco11present, releasing. The data for the next row is loaded into the columnsand a select pulse applied to that row and so on sequentially until theend of the display is reached. Addressing is then begun again with row0; however this time the addressing occurs within frame 1.

The difference between the frames is that the correspondence betweendata and column voltages is switched, a binary zero is now representedby Vco10, and the row select pulse is now at the level of Vsel F1. Usingthis technique, the overall polarity of the voltages applied to thedisplay array is alternated with each frame. This is useful, especiallyfor MEMS based displays, because it allows for the compensation of anyDC level charge buildup that can occur when only voltages of a singlepolarity are applied. The buildup of a charge within the structure cansignificantly offset the electroptical curve of the IMod or other MEMdevice.

Color Display Schemes

Because the IMod is a versatile device with a variety of potentialoptical responses, a number of different color display schemes areenabled having different attributes. One potential scheme exploits thefact that there are binary IMod designs that are capable of achievingcolor states, black states, and white states in the same IMod. Thiscapability can be used to achieve a color scheme that can be describedas “base+pigment”. This terminology is used because the approach isanalogous to the way in which paint colors are produced by addingpigments to a white base to achieve a desired color. Using thisapproach, a particular paint can attain any color in the spectrum andany level of saturation by controlling the content and amount ofpigments that are added to the base. The same can be said for a displaythat incorporates colored and black and white pixels.

As shown in FIG. 4A, a pixel 400 comprises five subpixel elements, 402,404, 406, and 408, with each subpixel capable of reflecting red, green,blue, and white respectively. All of the subpixels are capable of ablack state. Control over the brightness of each subpixel can beaccomplished using pulse width modulation related techniques asdiscussed in U.S. Pat. No. 5,835,255. In conjunction with properlyselected relative subpixel sizes, this results in a pixel over which avery large degree of control can be exercised of brightness andsaturation. For example, by minimizing the overall brightness of thewhite subpixels, highly saturated colors may be achieved. Conversely, byminimizing the brightness of the color subpixels, or by maximizing themin conjunction with the white subpixels, a bright black and white modemay be achieved. All variations in between are obviously attainable aswell.

User Control of Color Scheme

The previously described color scheme, as well as the inherentattributes of an IMod-based display in terms of resolution, gray scaledepth, and refresh rate, provides flexibility in display performance.Given this range, it is useful to give the user of a product containingsuch a display control over its general characteristics. Alternatively,it may be advantageous for the display to automatically adapt todifferent viewing needs.

For example, a user may want to use a product in black and white modeif, some context, only text were being viewed. In another situation,however, the user may want to view high quality color still images, orin yet another mode may want to view live video. Each of these modes,while potentially within the range of a given IMod displayconfiguration, requires tradeoffs in particular attributes. Tradeoffsinclude the need for low refresh rates if high-resolution imagery isrequired, or the ability to achieve high gray scale depth if only blackand white is requested.

To give the user this kind of on demand flexibility, the controllerhardware may be reconfigurable to some extent. Tradeoffs are aconsequence of the fact that any display has only a certain amount ofbandwidth, which is fundamentally limited by the response time of thepixel elements and thus determines the amount of information which canbe displayed at a given time.

One display architecture that could provide such flexibility isillustrated in FIG. 4B. In this block diagram, controller logic 412 isimplemented using one of a variety of IC technologies, includingprogrammable logic devices (PLAs) and field programmable gate arrays(FPGAs), which allow for the functionality of the component to bealtered or reconfigured after it leaves the factory. Such devices, whichare traditionally used for specialized applications such as digitalsignal processing or image compression, can provide the high performancenecessary for such processing, while supplying flexibility during thedesign stage of products incorporating such devices.

The controller 412 provides signals and data to the driver electronics414 and 416 for addressing the display 418. Conventional controllers arebased on IC's or Application Specific Integrated Circuits (ASICs), whichare effectively “programmed” by virtue of their design duringmanufacture. The term program, in this case, means an internal chiplayout comprising numerous basic and higher level logical components(logic gates and logic modules or assemblies of gates). By using fieldprogrammable devices such PLAs or FPGAs, different displayconfigurations may be loaded into the display controller component inthe form of hardware applications or “hardapps”, from a component 410,which could be memory or a conventional microprocessor and memory. Thememory could be in the form of EEPROMS or other reprogrammable storagedevices, and the microprocessor could take on the form of simplemicrocontroller whose function is to load the hardapp from memory intothe FPGA, unless this were performed by whatever processor is associatedwith the general functioning of the product. This approach isadvantageous because with relatively simple circuitry it is possible toachieve a wide variety of different display performance configurationsand mixed display scan rates, along with the potential to combine them.

One portion of the screen, for example, might be operated as alow-resolution text entry area, while another provides high qualityrendition of an incoming email. This could be accomplished, within theoverall bandwidth limitations of the display, by varying the refreshrate and # of scans for different segments of the display. Thelow-resolution text area could be scanned rapidly and only once or twicecorresponding to one or two bits of gray scale depth. The high renditionemail area could be scanned rapidly and with three or four passescorresponding to three or four bits of grayscale.

Configurable Electronic Products

This idea may be generalized to include not just the functionality ofthe display controller, but also the functionality of the overallproduct. FIG. 4C shows a configuration of a generic portable electronicproduct 418 that has a programmable logic device or equivalent at itscore 420. In many display centric personal electronic products, such asPDAs (personal digital assistants) and electronic organizers, thecentral processor is a variant of a RISC processor that uses a reducedinstruction set. While RISC processors are more efficient versions ofCPUs that power most personal computers, they are still general-purposeprocessors that expend a lot of energy performing repetitive tasks suchas retrieving instructions from memory.

In personal computers, power consumption is not an issue, and the usertypically wants to run a large number of complicated softwareapplications. The opposite is true of typical display centric/personalelectronic products. They are required to consume low power and offer arelatively small number of relatively simple programs. Such a regimefavors implementing the special purpose programs, which could includeweb browsers, calendar functions, drawing programs, telephone/addressdatabases, and handwriting/speech recognition among others, as hardapps.Thus whenever a particular mode of functionality, e.g., a program, isrequired by the user, the core processor is reconfigured with theappropriate hardapp and the user interacts with the product. Thus thehardapp processor, a variant of a Field Programmable Gate Array has thehardapp (i.e. program) manifested in its internal logic and connections,which get re-arranged and re-wired every time a new hardapp is loaded.Numerous suppliers of these components also provide an applicationdevelopment system that allows a specialized programming language (ahardware description language) to be reduced into the logicalrepresentation that makes up the appropriate processor. Numerous effortsare also underway to simplify the process or reducing higher levelprogramming languages into this form as well. One approach to realizingsuch a processor is detailed in the paper Kouichi Nagami, et al,“Plastic Cell Architecture: Towards Reconfigurable Computing forGeneral-Purpose”, Proc. IEEE Workshop on FPGA-based Custom ComputingMachines, 1998.

Referring again to FIG. 4C, the hardapp processor 420 is shown at thecenter of a collection of I/O devices and peripherals that it willutilize, modify, or ignore based on the nature and function of thehardapp currently loaded. The hardapps can be loaded from memory 422resident in the product, or from an external source via RF or IRinterface, 424, which could pull hardapps from the internet, cellularnetworks, or other electronic devices, along with content for aparticular hardapp application. Other examples of hardapps include voicerecognition or speech synthesis algorithms for the audio interface 432,handwriting recognition algorithms for pen input 426, and imagecompression and processing modes for image input device 430. Such aproduct could perform a myriad of functions by virtue of its majorcomponents, the display as the primary user interface and thereconfigurable core processor. Total power consumption for such a devicecould be on the order of tens of milliwatts versus the several hundredmilliwatts consumed by existing products.

Decoupling Electromechanical Aspects From Optical Aspects

U.S. patent application Ser. Nos. 08/769,947, abandoned, filed on Dec.19, 1996, and 09/056,975 filed on Apr. 4, 1998, and incorporated byreference, have described IMod designs that propose to decouple theelectromechanical performance of an IMod from its optical performance.Another way in which this may be accomplished is illustrated in FIGS. 5Aand 5B. This design uses electrostatic forces to alter the geometry ofan interferometric cavity. Electrode 502 is fabricated on substrate 500and electrically isolated from membrane/mirror 506 by insulating film504. Electrode 502 functions only as an electrode, not also as a mirror.

An optical cavity 505 is formed between membrane/mirror 506 andsecondary mirror 508. Support for secondary mirror 508 is provided by atransparent superstructure 510, which can be a thick deposited organic,such as SU-8, polyimide, or an inorganic material. With no voltageapplied, the membrane/mirror 506, maintains a certain position shown inFIG. 5A, relative to secondary mirror 508, as determined by thethickness of the sacrificial layers deposited during manufacture. For anactuation voltage of about four volts a thickness of several thousandangstroms might be appropriate. If the secondary mirror is made from asuitable material, say chromium, and the mirror/membrane made from areflective material such as aluminum, then the structure will reflectcertain frequencies of light 511 which may be perceived by viewer 512.In particular, if the chromium is thin enough to be semitransparent,about 40 angstroms, and the aluminum sufficiently thick, at least 500angstroms, as to be opaque, then the structure may have a wide varietyof optical responses. FIGS. 5C and 5D show examples of black and whiteand color responses respectively, all of which are determined by thecavity length, and the thickness of the constituent layers.

FIG. 5B shows the result of a voltage applied between primary electrode502 and membrane mirror 506. The membrane/mirror is vertically displacedthus changing the length of the optical cavity and therefore the opticalproperties of the IMod. FIG. 5C shows one kind of reflective opticalresponse which is possible with the two states, illustrating the blackstate 521 when the device is fully actuated, and a white state 523 whenthe device is not. FIG. 5D shows an optical response with color peaks525, 527, and 529, corresponding to the colors blue, green, and redrespectively. The electromechanical behavior of the device thus may becontrolled independently of the optical performance. Materials andconfiguration of the primary electrode, which influence theelectromechanics, may be selected independently of the materialscomprising the secondary mirror, because they play no role in theoptical performance of the IMod. This design may be fabricated usingprocesses and techniques of surface micromachining, for example, theones described in U.S. patent application Ser. No. 08,688,710, now U.S.Pat. No. 6,040,937, filed on Jul. 31, 1996 and incorporated byreference.

In another example, shown in FIG. 6A, the support structure for the IMod606 is positioned to be hidden by the membrane/mirror 608. In this waythe amount of inactive area is effectively reduced because the viewersees only the area covered by the membrane/mirror and the minimum spacebetween adjoining IMods. This is unlike the structure in FIG. 5A wherethe membrane supports are visible and constitute inactive andinaccurate, from a color standpoint, area. FIG. 6B, reveals the samestructure in the actuated state.

In FIG. 7A, another geometric configuration is illustrated for use in anIMod structure. This design is similar to one shown in U.S. Pat. No.5,638,084. That design relied upon an opaque plastic membrane that isanisotropically stressed so that it naturally resides in a curled state.Application of a voltage flattens the membrane to provide a MEMS-basedlight shutter.

The device's functionality may be improved by making it interferometric.The IMod variation is shown in FIG. 7A where thin film stack 704 is likethe dielectric/conductor/insulator stack which is the basis for theinduced absorber IMod design discussed in U.S. patent application Ser.No. 08/688,710, now U.S. Pat. No. 6,040,937, filed on Jul. 31, 1996 andincorporated by reference.

Application of a voltage between aluminum membrane 702 and stack 704causes the membrane 702 to lie flat against the stack. Duringfabrication, aluminum 702, which could also include other reflectivemetals (silver, copper, nickel), or dielectrics or organic materialswhich have been undercoated with a reflective metal, is deposited on athin sacrificial layer so that it may be released, using wet etch or gasphase release techniques. Aluminum membrane 702 is further mechanicallysecured to the substrate by a support tab 716, which is depositeddirectly on optical stack 704. Because of this, light that is incidenton the area where the tab and the stack overlap is absorbed making thismechanically inactive area optically inactive as well. This techniqueeliminates the need for a separate black mask in this and other IModdesigns.

Incident light 706 is either completely absorbed or a particularfrequency of light 708, is reflected depending on the spacing of thelayers of the stack. The optical behavior is like that of the inducedabsorber IMod described in U.S. patent application Ser. No. 08/688,710,now U.S. Pat. No. 6,040,937, filed on Jul. 31, 1996, and incorporated byreference.

FIG. 7B shows the device configuration when no voltage is applied. Theresidual stresses in the membrane induce it to curl up into a tightlywound coil. The residual stresses can be imparted by deposition of athin layer of material 718 on top of the membrane, which has extremelyhigh residual tensile stress. Chromium is one example in which highstresses may be achieved with a film thickness a low as several hundredangstroms. With the membrane no longer obstructing its path, light beam706 is allowed to pass through the stack 704 and intersect with plate710. Plate 710 can reside in a state of being either highly absorbing orhighly reflective (of a particular color or white). For the modulator tobe used in a reflective display, the optical stack 704 would be designedsuch that when the device is actuated it would either reflect aparticular color (if plate 710 were absorbing) or be absorbing (ifplate, 710 were reflective).

Rotational Actuation

As shown in FIG. 8A, another IMod geometry relies on rotationalactuation. Using the processes discussed in U.S. patent application Ser.No. 08/688,710, now U.S. Pat. No. 6,040,937, filed on Jul. 31, 1996 andincorporated by reference, electrode 802, an aluminum film about 1000angstroms thick, is fabricated on substrate 800. Support post 808 androtational hinge 810, support shutter 812, upon which a set ofreflecting films 813 has been deposited. The support shutter may be analuminum film which is several thousand angstroms thick. Its X-Ydimensions could be on the order of tens to several hundred microns. Thefilms may be interferometric and designed to reflect particular colors.A fixed interferometric stack in the form of an induced absorber likethat described in U.S. patent application Ser. No. 08/688,710, now U.S.Pat. No. 6,040,937, filed on Jul. 31, 1996 and incorporated by referencewould suffice. They may also comprise polymers infused with colorpigments, or they may be aluminum or silver to provide broadbandreflection. The electrode 802 and the shutter 812 are designed such thatthe application of a voltage (e.g., 10 volts) between the two causes theshutter 812 to experience partial or full rotation about the axis of thehinge. Only shutter 818 is shown in a rotated state although typicallyall of the shutters for a given pixel would be driven in unison by asignal on the common bus electrode 804. Such a shutter would experiencea form of electromechanical hysteresis if the hinges and electrodedistances were designed such that the electrostatic attraction of theelectrodes overcomes the spring tension of the hinge at some pointduring the rotation. The shutters would thus have twoelectromechanically stable states.

In a transmissive mode of operation, the shutter would either blockincident light or allow it to pass through. FIG. 8A illustrates thereflective mode where incident light 822 is reflected back to the viewer820. In this mode, and in one state, the shutter either reflects a whitelight, if the shutter is metallized, or reflects a particular color orset of colors, if it is coated with interferometric films or pigments.Representative thicknesses and resulting colors, for an interferometricstack, are also described in U.S. patent application Ser. No.08/688,710, now U.S. Pat. No. 6,040,937, filed on Jul. 31, 1996 andincorporated by reference. In the other state, the light is allowed topass through and be absorbed in substrate 800 if the opposite side ofthe shutter were coated with an absorbing film or films 722. These filmscould comprise another pigment infused organic film, or an inducedabsorber stack designed to be absorbing. Conversely, the shutters may behighly absorbing, i.e., black, and the opposite side of substrate 800coated with highly reflective films 824, or be selectively coated withpigment or interferometric films to reflect colors, along the lines ofthe color reflecting films described above.

Operation of the device may be further enhanced by the addition ofsupplementary electrode 814, which provides additional torque to theshutter when charged to a potential that induces electrostaticattraction between supplementary electrode 814 and shutter 812.Supplementary electrode 814 comprises a combination of a conductor 814and support structure 816. The electrode may comprise a transparentconductor such as ITO that could be about thousand angstroms thick. Allof the structures and associated electrodes are machined from materialsthat are deposited on the surface of a single substrate, i.e.monolithically, and therefore are easily fabricated and reliablyactuated due to good control over electrode gap spaces. For example, ifsuch an electrode were mounted on an opposing substrate, variations inthe surface of both the device substrate and opposing substrate couldcombine to produce deviations as much as several microns or more. Thusthe voltage required to affect a particular change in behavior couldvary by as much as several tens of volts or more. Structures that aremonolithic follow substrate surface variations exactly and suffer littlesuch variation.

FIG. 8B, steps 1-7, shows a fabrication sequence for the rotationalmodulator. In step 1, substrate 830 has been coated with electrode 832and insulator 834. Typical electrode and insulator materials arealuminum and silicon dioxide, each of a thickness of one thousandangstroms each. These are patterned in step 2. Sacrificial spacer 836, amaterial such as silicon several microns in thickness, has beendeposited and patterned in step 3 and coated with post/hinge/shuttermaterial 838 in step 4. This could be an aluminum alloy ortitanium/tungsten alloy about 1000 angstroms thick. In step 5, material838 has been patterned to form bus electrode 844, support post 840, andshutter 842. Shutter reflector 846 has been deposited and patterned instep 6. In step 7, the sacrificial spacer has been etched away yieldingthe completed structure. Step 7 also reveals a top view of the structureshowing detail of the hinge comprising support posts 848, torsion arm850, and shutter 852.

Switching Elements

For IMods that are binary devices only a small number of voltage levelsis required to address a display. The driver electronics need notgenerate analog signals that would be required to achieve gray scaleoperation.

Thus, the electronics may be implemented using other means as suggestedin U.S. patent application Ser. No. 08/769,947, filed on Dec. 19, 1996and incorporated by reference. In particular the drive electronics andlogic functions can be implemented using switch elements based on MEMS.

FIGS. 9A through 9E illustrate the concept. FIG. 9A is a diagram of abasic switch building block with an input 900 making a connection tooutput 904 by application of a control signal 902. FIG. 9B illustrateshow a row driver could be implemented. The row driver for the addressingscheme described above requires the output of three voltage levels.Application of the appropriate control signals to the row driver allowsone of the input 10 voltage levels to be selected for output 903. Theinput voltages are Vco11, Vco10, and Vbias corresponding to 906, 908,and 910 in the figure. Similarly, for the column driver shown in FIG.9C, the appropriate control signals result in the selection of one orthe other input voltage levels for delivery to the output 920. The inputvoltages are Vsel F1, Vsel F0, and ground, corresponding to 914, 916,and 918 in the figure. FIG. 9D illustrates how a logic device 932, maybe implemented, in this case a NAND gate, using basic switch buildingblocks 934, 936, 938, and 940. All of these components can be configuredand combined in a way that allows for the fabrication of the displaysubsystem shown in FIG. 9E. The subsystem comprises controller logic926, row driver 924, column driver 928, and display array 930, and usesthe addressing scheme described above in FIG. 3.

Fabrication of the switch elements as MEMS devices makes it possible tofabricate an entire display system using a single process. The switchfabrication process becomes a subprocess of the IMod fabrication processand is illustrated in FIG. 10A.

Step 1 shows both a side view and top view of the initial stage. Arrow1004 indicates the direction of the perspective of the side view.Substrate 1000 has had sacrificial spacer 1002 a silicon layer 2000angstroms thick deposited and patterned on its surface. In step 2, astructural material, an aluminum alloy several microns thick, has beendeposited and patterned to form source beam 1010, drain structure 1008,and gate structure 1006. Several hundred angstroms of a non-corrodingmetal such as gold, iridium or platinum may be plated onto thestructural material at this point to maintain low contact resistancethrough the life of the switch. Notch 1012 has been etched in sourcebeam 1010 to facilitate the movement of the beam in a plane parallel tothat of the substrate. The perspective of the drawing is different insteps 3 and 4, which now compare a front view with a top view. Arrows1016 indicate the direction of the perspective of the front view. Instep 3, the sacrificial material has been etched away leaving the sourcebeam 1010 intact and free to move.

When a voltage is applied between the source beam and the gatestructure, the source beam 1010 is deflected towards gate 1006 until itcomes into contact with the drain 1008, thereby establishing electricalcontact between the source and the drain. The mode of actuation isparallel to the surface of the substrate, thus permitting a fabricationprocess that is compatible with the main IMod fabrication processes.This process also requires fewer steps than those used to fabricateswitches that actuate in a direction normal the substrate surface.

FIG. 10B and 10C illustrates two alternative designs for planar MEMswitches. The switch in FIG. 10B differs in that switch beam 1028 servesto provide contact between drain 1024 and source 1026. In the switch ofFIG. 10A, currents that must pass through the source beam to the drainmay effect switching thresholds, complicating the design of circuits.This is not the case with switch 1020. The switch in FIG. 10C reveals afurther enhancement. In this case, insulator 1040 electrically isolatesswitch beam 1042 from contact beam 1038. This insulator may be amaterial such as SiO2 that can be deposited and patterned usingconventional techniques. Use of such a switch eliminates the need toelectrically isolate switch drive voltages from logic signals incircuits comprising these switches.

Multidimensional Photonic Structures

In general, IMods feature elements that have useful optical propertiesand are movable by actuation means with respect to themselves or otherelectrical, mechanical or optical elements.

Assemblies of thin films to produce interferometric stacks are a subsetof a larger class of structures that we shall refer to asmultidimensional photonic structures. Broadly, we define a photonicstructure as one that has the ability to modify the propagation ofelectromagnetic waves due to the geometry and associated changes in therefractive index of the structure. Such structures have a dimensionalaspect because they interact with light primarily along one or moreaxes. Structures that are multidimensional have also been referred to asphotonic bandgap structures (PBG's) or photonic crystals. The text“Photonic Crystals” by John D. Joannopoulos, et al describes photonicstructures that are periodic.

A one-dimensional PBG can occur in the form of a thin film stack. By wayof example, FIG. 16 shows the fabrication and end product of an IMod inthe form of a dielectric Fabry-Perot filter. Thin film stacks 1614 and1618, which could be alternating layers of silicon and silicon dioxideeach a quarter wave thick, have been fabricated on a substrate to forman IMod structure that incorporates central cavity 1616. In general, thestack is continuous in the X and Y direction, but has a periodicity inthe optical sense in the Z direction due to variations in the refractiveindex of the material as they are comprised of alternating layers withhigh and low indices. This structure can be considered one-dimensionalbecause the effect of the periodicity is maximized for waves propagatingalong one axis, in this case the Z-axis.

FIGS. 11A and 11B illustrate two manifestations of a two-dimensionalphotonic structure. In FIG. 11A, a microring resonator 1102 can befabricated from one of a large number of well known materials, an alloyof tantalum pentoxide and silicon dioxide for example, using well knowntechniques. For a device optimized for wavelengths in the 1.55 um range,typical dimensions are w=1.5 um, h=1.0 um, and r=10 um.

Fabricated on a substrate 1100 (glass is one possibility though thereare many others), the structure is essentially a circular waveguidewhose refractive index and dimensions w, r, and h determine thefrequencies and modes of light which will propagate within it. Such aresonator, if designed correctly, can act as a frequency selectivefilter for broadband radiation that is coupled into it. In this case,the radiation is generally propagating in the XY plane as indicated byorientation symbol 1101. The one-dimensional analog of this device wouldbe a Fabry-Perot filter made using single layer mirrors. Neither deviceexhibits a high order optical periodicity, due to the single layer“boundaries” (i.e. mirrors); however, they can be considered photonicstructures in the broad sense.

A more traditional PBG is shown in FIG. 11B. Columnar array 1106presents a periodic variation in refractive index in both the X and Ydirections. Electromagnetic radiation propagating through this medium ismost significantly affected if it is propagating within the XY plane,indicated by orientation symbol 1103.

Because of its periodic nature, the array of FIG. 11B shares attributeswith a one-dimensional thin film stack, except for its higher-orderdimensionality. The array is periodic in the sense that along some axisthrough the array, within the XY plane, the index of refraction variesbetween that of the column material, and that of the surroundingmaterial, which is usually air. Appropriate design of this array,utilizing variations on the same principles applied to the design ofthin film stacks, allows for the fabrication of a wide variety ofoptical responses, (mirrors, bandpass filters, edge filters, etc.)acting on radiation traveling in the XY plane. Array 1106 in the caseshown in FIG. 11B includes a singularity or defect 1108 in the form of acolumn that differs in its dimension and/or refractive index. Forexample, the diameter of this column might be fractionally larger orsmaller than the remaining columns (which could be on the order of aquarter wavelength in diameter), or it may be of a different material(perhaps air vs. silicon dioxide). The overall size of the array isdetermined by the size of the optical system or component that needs tobe manipulated. The defect may also occur in the form of the absence ofa column or columns (a row), depending on the desired behavior. Thisstructure is analogous to the dielectric Fabry-Perot filter of FIG. 16,but it functions in only two dimensions. In this case, the defect isanalogous to the cavity, 1616. The remaining columns are analogous tothe adjacent two-dimensional stacks.

The relevant dimensions of the structure of FIG. 11B are denoted bycolumn x spacing sx, column y spacing sy, (either of which could beconsidered the lattice constant), column diameter d, and array height,h. Like the quarter wave stack, the one-dimensional equivalent, columndiameters and spacings can be on the order of a quarter wave. Theheight, h, is determined by the desired propagation modes, with littlemore than one half wavelength used for single mode propagation. Theequations for relating the size of the structures to their effect onlight are well known and documented in the text “Photonic Crystals” byJohn D. Joannopoulos, et al.

This kind of structure may also be fabricated using the same materialsand techniques used to fabricate the resonator 1102. For example, asingle film of silicon may be deposited on a glass substrate andpatterned, using conventional techniques, and etched using reactive ionetching to produce the high aspect ratio columns. For a wavelength of1.55 um, the diameter and spacing of the columns could be on the orderof 0.5 um and 0.1 um respectively.

Photonic structures also make it possible to direct radiation underrestrictive geometric constraints. Thus they are quite useful inapplications where it is desirable to redirect and/or select certainfrequencies or bands of frequencies of light when dimensionalconstraints are very tight. Waveguides, channeling light propagating inthe XY plane, may be fabricated which can force light to make 90 degreeturns in a space less than the wavelength of the light. This can beaccomplished, for example, by creating the column defect in the form ofa linear row, which can act as the waveguide.

A three-dimensional structure is illustrated in FIG. 12.Three-dimensional periodic structure 1202 acts on radiation propagatingin the XY, YZ, and XZ planes. A variety of optical responses may beattained by appropriate design of the structure and selection of itsconstituent materials. The same design rules apply, however they areapplied three-dimensionally here. Defects occur in the form of points,lines, or regions, vs. points and lines, which differ in size and/orrefractive index from the surrounding medium. In FIG. 12, the defect1204 is a single point element but may also be linear or a combinationof linear and point elements or regions. For example, a “linear” or“serpentine” array of point defects may be fabricated such that itfollows an arbitrary three-dimensional path through the PBG, and acts asa tightly constrained waveguide for light propagating within it. Thedefect would generally be located internally but is shown on the surfacefor purposes of illustration. The relevant dimensions of this structureare all illustrated in the figure. The diameter and spacing andmaterials of the PBG are completely application dependent, however theaforementioned design rules and equations also apply.

Three-dimensional PBGs are more complicated to make. Conventional meansfor fabricating one-dimensional or two-dimensional features, if appliedin three dimensions, would involve multiple applications of deposition,pattern, and etch cycles to achieve the third dimension in thestructure. Fabrication techniques for building periodicthree-dimensional structures include: holographic, where aphotosensitive material is exposed to a standing wave and replicates thewave in the form of index variations in the material itself;self-organizing organic or self-assembling materials that rely on innateadhesion and orientation properties of certain co-polymeric materials tocreate arrays of columnar or spherical structures during the depositionof the material; ceramic approaches that can involve the incorporationof a supply of spherical structures of controlled dimensions into aliquid suspension that, once solidified, organizes the structures, andcan be removed by dissolution or high temperature; combinations of theseapproaches; and others.

Co-polymeric self-assembly techniques are especially interesting becausethey are both low temperature and require minimal or nophotolithography. In general, this technique involves the dissolution ofa polymer, polyphenylquinoine-block-polystyrene (PPQmPSn) is oneexample, into a solvent such as carbon disulfide. After spreading thesolution onto a substrate and allowing the solvent to evaporate, a closepacked hexagonal arrangement of air filled polymeric spheres results.The process can be repeated multiple times to produce multilayers, theperiod of the array may be controlled by manipulating the number ofrepeat units of the components (m and n) of the polymer. Introduction ofa nanometer sized colloid comprising metals, oxides, or semiconductorsthat can have the effect of reducing the period of the array further, aswell as increasing the refractive index of the polymer.

Defects may be introduced via direct manipulation of the material on asubmicron scale using such tools as focused ion beams or atomic forcemicroscopes. The former may be used to remove or add material in verysmall selected areas or to alter the optical properties of the material.Material removal occurs when the energetic particle beam, such as thatused by a Focused Ion Beam tool, sputters away material in its path.Material addition occurs when the focused ion beam is passed through avolatile metal containing gas such as tungsten hexafluoride (fortungsten conductor) or silicon tetrafluoride (for insulating silicondioxide). The gas breaks down, and the constituents are deposited wherethe beam contacts the substrate. Atomic force microscopy may be used tomove materials around on the molecular scale.

Another approach involves the use of a technique that can be calledmicro-electrodeposition and which is described in detail in U.S. Pat.No. 5,641,391. In this approach a single microscopic electrode can beused to define three-dimensional features of submicron resolution usinga variety of materials and substrates. Metal “defects” deposited in thisway could be subsequently oxidized to form an dielectric defect aroundwhich the PBG array could be fabricated using the techniques describedabove.

The existence of surface features, in the form of patterns of othermaterials, on the substrate upon which the PBG is fabricated may alsoserve as a template for the generation of defects within the PBG duringits formation. This is particularly relevant to PBG processes that aresensitive to substrate conditions, primarily self-assembly approaches.These features may encourage or inhibit the “growth” of the PBG in ahighly localized region around the seed depending on the specific natureof the process. In this way, a pattern of defect “seeds” may be producedand the PBG formed afterwards with the defects created within during thePBG formation process.

Thus, the class of devices known as IMods may be further broadened byincorporating the larger family of multidimensional photonic structuresinto the modulator itself. Any kind of photonic structure, which isinherently a static device, may now be made dynamic by altering itsgeometry and/or altering its proximity to other structures. Similarly,the micromechanical Fabry-Perot filter (shown in FIG. 16), comprisingtwo mirrors which are each one-dimensional photonic structures, may betuned by altering the cavity width electrostatically.

FIG. 13 shows two examples of IMod designs incorporating two-dimensionalPBGs. In FIG. 13A, a cutaway diagram reveals a self-supporting membrane1304, which has been fabricated with a microring resonator 1306 mountedon the side facing the substrate. Waveguides 1301 and 1302 lying withinthe bulk of the substrate 1303 are planar and parallel and can befabricated using known techniques. In FIG. 13A, the IMod is shown in theun-driven state with a finite airgap (number) between the microring andthe substrate. The microring is fabricated so that its position overlapsand aligns with the paired waveguides in the substrate below. Dimensionsof the microring are identical to the example described above.Crossection 1305 shows the dimensions of the waveguides which could bew=1 um, h=0.5 um, and t=100 nm. In the un-driven state, light 1308,propagates undisturbed in waveguide 1302, and the output beam 1310 isspectrally identical to the input 1308.

Driving the IMod to force the microring into intimate contact with thesubstrate and waveguides alters the optical behavior of the device.Light propagating in waveguide 1302 may now couple into the microring bythe phenomenon of evanescence. The microring, if sized appropriately,acts as an optical resonator coupling a selected frequency fromwaveguide 1302 and injecting it into waveguide 1301. This is shown inFIG. 13B where light beam 1312 is shown propagating in a directionopposite the direction of light 1308. Such a device may be used as afrequency selective switch that picks particular wavelengths out of awaveguide by the application of a voltage or other driving meansrequired to bring the structure into intimate contact with theunderlying waveguides. A static version of this geometry is described inthe paper B. E. Little, et al, “Vertically Coupled Microring ResonatorChannel Dropping Filter”, IEEE Photonics Technology Letters, vol. 11,no. 2, 1999.

Another example is illustrated in FIG. 13C. In this case, a pair ofwaveguides 1332 and 1330 and resonator 1314 are fabricated on thesubstrate in the form of a columnar PBG. The PBG is a uniform array ofcolumns, with the waveguides defined by removing two rows (one for eachwaveguide), and the resonator defined by removing two columns. Top view1333 provides more detail of the construction of waveguides 1330 and1332, and the resonator 1314. Dimensions are dependent on the wavelengthof interest as well as materials used. For a wavelength of 1.55 um, thediameter and spacing of the columns could be on the order of 0.5 um and1 um respectively. The height, h, determines the propagation modes whichwill be supported and should be slightly more than half the wavelengthif only single modes are to be propagated.

On the inner surface of the membrane 1315 are fabricated two isolatedcolumns 1311, which are directed downwards, and have the same dimensionsand are of the same material (or optically equivalent) as the columns onthe substrate. The resonator and columns are designed to complement eachother; there is a corresponding absence of a column in the resonatorwhere the column on the membrane is positioned.

When the IMod is in an undriven state, there is a finite vertical airgap1312, of at least several hundred nanometers between the PBG and themembrane columns and therefore no optical interaction occurs. Theabsence of columns in the resonator act like defects, causing couplingbetween waveguides 1330 and 1332. In this state the device acts as doesthe one shown in FIG. 13B and selected frequencies of light propagatingalong waveguide are now injected into waveguide 1332, and propagate inthe opposite direction in the form of light 1329.

Driving the IMod into contact with the PBG, however, places the columnsinto the resonator altering its behavior. The defects of the resonatorare eliminated by the placement of the membrane columns. The device inthis state acts as does the one shown in FIG. 13A, with light 1328propagating without interference.

A static version of this geometry is described in the paper H. A. Haus“Channel drop filters in photonic crystals”, Optics Express, vol. 3,no.1, 1998.

Optical Switches

In FIG. 14A, a device based on the induced absorber includes aself-supporting aluminum membrane 1400, on the order of tens to hundredsof microns square, which is suspended over a stack of materials 1402comprising a combination of metals and oxides and patterned ontransparent substrate. The films utilized in the induced absorbermodulator, described in U.S. patent application Ser. No. 08/688,710, nowU.S. Pat. No. 6,040,937, filed on Jul. 31, 1996, and incorporated byreference, could serve this purpose. The films on the substrate may alsocomprise a transparent conductor, such as ITO. The structure mayincorporate on its underside a lossy metal film such as molybdenum ortungsten, of several hundred angstroms in thickness.

The materials are configured so that in the undriven state the devicereflects in a particular wavelength region, but becomes very absorbingwhen the membrane is driven into contact. Side view 1410 shows a view ofthe device looking into the side of the substrate. Light beam 1408propagates at some arbitrary angle through the substrate and is incidenton IMod 1406, shown in the un-driven state. Assuming the frequency ofthe light corresponds with the reflective region of the IMod in theun-driven state, the light is reflected at a complementary angle andpropagates away. Side view, 1414, shows the same IMod in the drivenstate. Because the device is now very absorbing, the light which isincident upon it is no longer reflected but absorbed by the materials inthe IMod's stack.

Thus, in this configuration, the IMod may act as an optical switch forlight that is propagating within the substrate upon which it isfabricated. The substrate is machined to form surfaces that are highlypolished, highly parallel (to within {fraction (1/10)} of a wavelengthof the light of interest), and many times thicker (at least hundreds ofmicrons) than the wavelength of light. This allows the substrate to actas a substrate/waveguide in that light beams propagate in a directionwhich is, on average, parallel to the substrate but undergo multiplereflections from one surface to another. Light waves in such a structureare often referred to as substrate guided waves.

FIG. 14B shows a variation on this theme. Membrane 1420 is patternedsuch that it is no longer rectangular but is tapered towards one end.While the mechanical spring constant of the structure remains constantalong this length, electrode area decreases. Thus the amount of forcewhich can be applied electrostatically is lower at the narrower end ofthe taper. If a gradually increasing voltage is applied, the membranewill begin to actuate at the wider end first and actuation will progressalong arrow 1428 as the voltage increases.

To incident light, the IMod operates as an absorbing region whose areadepends on the value of the applied voltage. Side view 1434 shows theeffect on a substrate propagating beam when no voltage is applied. Thecorresponding reflective area 1429, which shows the IMod from theperspective of the incident beam, shows “footprint” 1431 of the beamsuperimposed on the reflective area. Since the entire area 1429 isnon-absorbing, beam, 1430, is reflected from IMod 1428 (with minimallosses) in the form of beam 1432.

In side view 1436, an interim voltage value is applied and the reflectedbeam 1440 has been attenuated to some extent because the reflectivearea, shown in 1437 is now partially absorbing. Views 1438 and 1429reveal the result of full actuation and the complete attenuation of thebeam.

Thus, by using a tapered geometry a variable optical attenuator may becreated, the response of which is directly related to the value of theapplied voltage.

Another kind of optical switch is illustrated in FIG. 15A. Support frame1500 is fabricated from a metal, such as aluminum several thousandangstroms in thickness, in such a way that it is electrically connectedto mirror 1502. Mirror 1502 resides on transparent optical standoff1501, which is bonded to support 1500. Mirror 1502 may comprise a singlemetal film or combinations of metals, oxides, and semiconducting films.

The standoff is fabricated from a material that has the same or higherindex of refraction than that of the substrate. This could be SiO2 (sameindex) or a polymer whose index can be varied. The standoff is machinedso that the mirror is supported at an angle of 45 degrees. Machining ofthe standoff can be accomplished using a technique known as analoglithography that relies on a photomask whose features are continuouslyvariable in terms of their optical density. By appropriate variation ofthis density on a particular feature. three-dimensional shapes can beformed in photoresist that is exposed using this mask. The shape canthen be transferred into other materials via reactive ion etching. Theentire assembly is suspended over conductor, 1503, which has beenpatterned to provide an unobstructed “window” 1505 into the underlyingsubstrate, 1504. That is to say the bulk of conductor 1503 has beenetched away so that window 1505, comprising bare glass, is exposed. Theswitch, like other IMods, can be actuated to drive the whole assemblyinto contact with the substrate/waveguide. Side view, 1512, shows theoptical behavior. Beam 1510 is propagating within the substrate at anangle 45 degrees from normal that prevents it from propagating beyondthe boundaries of the substrate. This is because 45 degrees is above theangle known as the critical angle, which allows the beam to be reflectedwith minimal or no losses at the interface 1519 between the substrateand the outside medium by the principle of total internal reflection(TIR).

The principle of TIR depends on Snell's law, but a basic requirement isthat the medium outside the substrate have an index of refraction thatis lower than that of the substrate. In side view, 1512, the device isshown with the switch 1506 in the un-driven state, and beam 1510propagating in an unimpeded fashion. When switch 1506 is actuated intocontact with the substrate as shown in side view 1514, the beam's pathis altered. Because the standoff has a refractive index greater than orequal to that of the substrate, the beam no longer undergoes TIR at theinterface. The beam propagates out of the substrate into the opticalstandoff, where it is reflected by the mirror. The mirror is angled, at45 degrees, such that the reflected beam is now traveling at an anglewhich is normal to the plane of the substrate. The result is that thelight may propagate through the substrate interface because it no longermeets the criteria for TIR, and can be captured by a fiber coupler 1520,which has been mounted on the opposite side of the substrate/waveguide.A similar concept is described in the paper, X. Zhou, et al, “WaveguidePanel Display Using Electromechanical Spatial Modulators”, SID Digest,vol. XXIX, 1998. This particular device was designed for emissivedisplay applications. The mirror may also be implemented in the form ofa reflecting grating, which may be etched into the surface of thestandoff using conventional patterning techniques. This approach,however, exhibits wavelength dependence and losses due to multiplediffraction orders that are not an issue with thin film mirrors.Additionally, alternative optical structures may be substituted for themirror as well with their respective attributes and shortcomings. Thesecan be categorized as refractive, reflective, and diffractive and caninclude micro-lenses (both transmissive and reflective), concave orconvex mirrors, diffractive optical elements, holographic opticalelements, prisms, and any other form of optical element which can becreated using micro-fabrication techniques. In the case where analternative optical element is used, the standoff and the angle itimparts to the optic may not be necessary depending on the nature of themicro-optic.

This variation on the IMod acts as a de-coupling switch for light.Broadband radiation, or specific frequencies if the mirror is designedcorrectly, can be coupled out of the substrate/waveguide at will. Sideview 1526 shows a more elaborate implementation in which an additionalfixed mirror, angled at 45 degrees, has been fabricated on the side ofthe substrate opposite that of the de-coupling switch. This mirrordiffers from the switch in that it cannot be actuated. By carefulselection of the angles of the mirrors on both structures, light 1522that has been effectively decoupled out of the substrate by switch 1506may be recoupled back into the substrate by re-coupling mirror 1528.However, by fabricating the recoupling mirror with differentorientations in the XY plane, the mirror combination may be used toredirect light in any new direction within the substrate/waveguide. Thecombination of these two structures will be referred to as a directionalswitch. Re-coupling mirrors can also be used to couple any light that ispropagating into the substrate in a direction normal to the surface.

FIG. 15B shows one implementation of an array of directional switches.Looking down onto the substrate 1535, linear array 1536 is an array offiber couplers which directs light into the substrate at an angle normalto the XY plane. An array of re-coupling mirrors (not visible) ispositioned directly opposite the fiber coupler array to couple lightinto the substrate parallel to beam 1530. On the surfaces of thesubstrate, 1535, are fabricated an array of directional switches ofwhich 1531 is one. The switches are positioned in a way such that lightcoupled into the substrate from any one of the input fiber couplers 1536may be directed to any one of the output fiber couplers 1532. In thisway the device may act as an N X N optical switch that can switch anyone of any number of different inputs to any one of any number ofdifferent outputs.

Tunable Filter

Returning to FIG. 16, an IMod in the form of a tunable Fabry-Perotfilter is shown. In this case, conducting contact pad 1602 has beendeposited and patterned along with dielectric mirrors 1604 and 1608 andsacrificial layer 1606. This may consist of a silicon film with athickness of some multiple of one-half a wavelength. The mirrors maycomprise stacks of materials, TiO2 (high index) and SiO2 (low index)being two examples, with alternating high and low indices, and one ofthe layers may also be air. Insulating layer 1610 is deposited andpatterned such that second contact pad 1612 only contacts mirror 1608.Mirror, 1608 is subsequently patterned leaving a mirror “island” 1614connected by supports 1615. The lateral dimensions of the island areprimarily determined by the size of light beam with which it willinteract. This is usually on the order of tens to several hundredmicrons. Sacrificial layer 1606 is partially etched chemically, butleaving standoffs of sufficient size to provide mechanical stability,probably on the order of tens of microns square. If the top layer ofmirror 1608 and the bottom layer of mirror 1604 are lightly doped to beconducting, then application of a voltage between contact pads 1602 and1612 will cause the mirror island to be displaced. Thus, the structure'soptical response may be tuned.

FIG. 17A shows an application of this tunable filter. On the top surfaceof substrate 1714 has been fabricated tunable filter 1704, mirrors 1716,and anti-reflection coating 1712. A mirror 1717 has also been fabricatedon the bottom surface of the substrate, e.g., from a metal such as goldof at least 100 nm thick. Mounted on the top surface of the substrate isan optical superstructure, 1706, whose inner surface is at least 95%reflective, e.g., by the addition of a reflecting gold film, and whichalso supports an angled mirror, 1710. In this device, light beam 1702propagates within the substrate at some angle that is larger than thecritical angle, which is approximately 41 degrees for a substrate ofglass and a medium of air. Therefore the mirrors 1716 are required tokeep it bounded within the confines of the substrate/waveguide. Thisconfiguration allows greater flexibility in the selection of angles atwhich the light propagates.

Beam 1702 is incident upon Fabry-Perot 1704, which transmits aparticular frequency of light 1708 while reflecting the rest 1709. Thetransmitted frequency is incident onto and reflected from the reflectivesuperstructure 1706, and is reflected again by mirror 1716 onto angledmirror 1710. Mirror 1710 is tilted such that the light is directedtowards antireflection coating 1712 at a normal angle with respect tothe substrate, and passes through and into the external medium. Thedevice as a whole thus acts as a wavelength selective filter.

The superstructure may be fabricated using a number of techniques. Onewould include the bulk micromachining of a slab of silicon to form acavity of precise depth, e.g., on the order of the thickness of thesubstrate and at least several hundred microns. The angled mirror isfabricated after cavity etch, and the entire assembly is bonded to thesubstrate, glass for example, using any one of many silicon/glassbonding techniques.

FIG. 17B is a more elaborate version. In this example, a second tunablefilter 1739 has been added to provide an additional frequency selectionchannel. That is to say that two separate frequencies may now beselected independently. Detectors 1738 have also been added to allow fora higher degree of integrated functionality.

FIG. 17C incorporates integrated circuits. Light beam 1750 has beencoupled into substrate 1770 and is incident upon tunable filter 1752.This filter is different than those of FIGS. 17A and 17B in that itincludes recoupling mirror 1756 that has been fabricated on the surfaceof the movable mirror of the filter. The angle of the mirror is suchthat the frequency selected by filter 1752 is now coupled directly backinto the substrate at a normal angle in the form of light beam 1758. Theremaining frequencies contained in light beam 1750 propagate until theyencounter recoupling mirror 1760 which is angled so that it presents asurface which is perpendicular to propagating beam 1756. The beam thusretraces its path back out of the device where it may be used by otherdevices that are connected optically. Light beam 1758 is incident on IC1764 that can detect and decode the information within this beam. ThisIC may be in the form of an FPGA or other silicon, silicon/germanium, orgallium aresenide device based integrated circuit that could benefitfrom being directly coupled to information carrying light. For example,a high bandwidth optical interconnect may be formed between ICs 1764 and1762 by virtue of the bidirectional light path 1772. This is formed by acombination of mirrors 1766 and recoupling mirrors 1768. Light can beemmitted by either ICs if they incorporate components such as verticalcavity surface emitting lasers (VCSELS) or light emitting diodes LEDs.Light can be detected by any number of optically sensitive components,with the nature of the component depending on the semiconductortechnology used to fabricate the IC. Light that is incident on the ICmay also be modulated by IMods that have been fabricated on the surfaceof the IC that is exposed to the substrate propagating light.

Optical mixer using substrate waveguide

FIGS. 18A and 18B are an illustration of a two-channel optical mixerimplemented using a TIR version of a substrate/waveguide. FIG. 18A showsa schematic of the device. Light containing multiple wavelengths has twoparticular wavelengths, 1801 and 1803, split off and directed towardstwo independent variable attentuators 1805. They are then output toseveral possible channels 1807 or into an optical stop 1813.

FIG. 18B reveals an implementation. The input light is directed into thedevice through fiber coupler 1800, through anti-reflection coating 1802,and coupled into the substrate using re-coupling mirror 1806. Therecoupling mirror directs the light onto tunable filter 1808, splittingoff frequency al (beam 1815) and all non-selected frequencies aredirected toward a second tunable filter 1809, which splits off frequencyλ2 (beam 1817), with the remaining frequencies, beam 1819, propagatingfurther downstream via TIR. Following the path of beam 1815, which wastransmitted by tunable filter 1808, the light is redirected back intothe substrate waveguide via mirror 1810, through an AR coating, andre-coupled back into the substrate. The re-coupling mirror 1811 directsbeam 1815 towards attenuator 1812 where it continues along a parallelpath with beam 1817 selected by the second tunable filter 1809. Thesetwo beams are positionally shifted by virtue of beam repositioner 1816.

This structure produces the same result as a recoupling mirror, exceptthat the mirror is parallel to the surface of the substrate. Because themirror is suspended a fixed distance beyond the substrate surface, theposition of the point of incidence on the opposite substrate interfaceis shifted towards the right. This shift is directly determined by theheight of the repositioner. The beam 1819, containing the unselectedwavelengths, is also shifted by virtue of repositioner 1818. The resultis that all three beams are equally separated when they are incident onan array of decoupling switches 1820 and 1824. These serve selectivelyto redirect the beams into one of two optical combiners, 1828 being oneof them or into detector/absorber 1830. The optical combiners may befabricated using a variety of techniques. A polymeric film patternedinto the form of a pillar with its top formed into a lens using reactiveion etching is one approach. The absorber/detector, comprising asemiconductor device that has been bonded to the substrate, serves toallow the measurement of the output power of the mixer. Opticalsuperstructures 1829 support external optical components and provide ahermetic package for the mixer.

The combination of planar IMods and a substrate waveguide provide afamily of optical devices that are easily fabricated, configured, andcoupled to the outside world because the devices reside on the waveguideand/or on the superstructure and are capable of operating on light whichis propagating within the waveguide, and between the waveguide and thesuperstructure. Because all of the components are fabricated in a planarfashion, economies of scale can be achieved by bulk fabrication overlarge areas, and the different pieces maybe aligned and bonded easilyand precisely. In addition, because all of the active components exhibitactuation in a direction normal to the substrate, they are relativelysimple to fabricate and drive, compared to more elaborate non-planarmirrors and beams. Active electronic components may be bonded to eitherthe superstructure or the substrate/waveguide to increase functionality.Alternatively, active devices may be fabricated as a part of thesuperstructure, particularly if it is a semiconductor such as silicon orgallium arsenide.

Printing Style Fabrication Processes

Because they are planar and because many of the layers do not requiresemiconducting electrical characteristics that require specializedsubstrates, IMods, as well as many other MEM structures, may takeadvantage of manufacturing techniques which are akin to those of theprinting industry. These kinds of processes typically involve a“substrate” which is flexible and in the form of a continuous sheet ofsay paper or plastic. Referred to as web fed processes, they usuallyinvolve a continuous roll of the substrate material which is fed into aseries of tools, each of which selectively coats the substrate with inkin order to sequentially build up a full color graphical image. Suchprocesses are of interest due to the high speeds with which product canbe produced.

FIG. 19 is a representation of such a sequence applied to thefabrication of a single IMod and, by extension, to the fabrication ofarrays of IMods or other microelectromechanical structures. Web source1900 is a roll of the substrate material such as transparent plastic. Arepresentative area 1902 on a section of material from the rollcontains, for the purposes of this description, only a single device.Embossing tool 1904 impresses a pattern of depressions into the plasticsheet. This can be accomplished by a metal master which has theappropriate pattern of protrusions etched on it.

The metal master is mounted on a drum that is pressed against the sheetwith enough pressure to deform the plastic to form the depressions. View1906 illustrates this. Coater 1908 deposits thin layers of materialusing well known thin film deposition processes, such as sputtering orevaporation. The result is a stack 1910 of four films comprising anoxide, a metal, an oxide, and a sacrificial film. These materialscorrespond to the induced absorber IMod design. A tool 1912 dispenses,cures, and exposes photoresist for patterning these layers. Once thepattern has been defined, the film etching occurs in tool 1914.Alternatively, patterning may be accomplished using a process known aslaser ablation. In this case, a laser is scanned over the material in amanner that allows it to be synchronized with the moving substrate. Thefrequency and power of the laser is such that it can evaporate thematerials of interest to feature sizes that are on the order of microns.The frequency of the laser is tuned so that it only interacts with thematerials on the substrate and not the substrate itself. Because theevaporation occurs so quickly, the substrate is heated only minimally.

In this device example, all of the films are etched using the samepattern. This is seen in 1918 where the photoresist has been strippedaway after the application of tool 1916. Tool 1920, is anotherdeposition tool that deposits what will become the structural layer ofthe IMod. Aluminum is one candidate for this layer 1922. This materialmay also include organic materials which exhibit minimal residual stressand which may be deposited using a variety of PVD and PECVD techniques.This layer is subsequently patterned, etched, and stripped ofphotoresist using tools 1924, 1926, and 1928 respectively. Tool 1930 isused to etch away the sacrificial layer. If the layer is silicon, thiscan be accomplished using XeF2, a gas phase etchant used for suchpurposes. The result is the self-supporting membrane structure 1932 thatforms the IMod.

Packaging of the resulting devices is accomplished by bonding flexiblesheet 1933 to the top surface of the substrate sheet. This is alsosupplied by a continuous roll 1936 that has been coated with a hermeticfilm, such as a metal, using coating tool 1934. The two sheets arejoined using bonding tool 1937, to produce the resulting packaged device1940.

Stress Measurement

Residual stress is a factor in the design and fabrication of MEMstructures. In IMods, and other structures in which structural membershave been mechanically released during the fabrication process, theresidual stress determines the resulting geometry of the member.

The IMod, as an interferometric device, is sensitive to variations inthe resulting geometry of the movable membrane. The reflected, or inother design cases transmitted, color is a direct function of the airgapspacing of the cavity. Consequently, variations in this distance alongthe length of a cavity can result in unacceptable variations in color.On the other hand, this property is a useful tool in determining-theresidual stress of the structure itself, because the variations in thecolor can be used to determine the variations and degree of deformationin the membrane. Knowing the deformed state of any material allows for adetermination of the residual stresses in the material. Computermodeling programs and algorithms can use two-dimensional data on thedeformation state to determine this. Thus the IMod structure can providea tool for making this assessment.

FIGS. 20A and 20B show examples of how an IMod may be used in thisfashion. IMods, 2000, and 2002, are shown from the perspective of theside and the bottom (i.e. viewed through the substrate). They are of adouble cantilever and single cantilever form respectively. In this case,the structural material has no residual stresses, and both membranesexhibit no deformation. As viewed through the substrate, the devicesexhibit a un form color that is determined by the thickness of thespacer layer upon which they were formed. IMods 2004 and 2006 are shownwith a stress gradient that is more compressive on the top than it is onthe bottom. The structural membranes exhibit a deformation as a result,and the bottom view reveals the nature of the color change that wouldresult. For example if color region 2016 were green, then color region2014 might be blue because it is closer to the substrate. Conversely,color region 2018 (shown on the double cantilever) might be red becauseit is farther away. IMods 2008 and 2010 are shown in a state where thestress gradient exhibits higher tensile stress on the top than on thebottom. The structural members are deformed appropriately, and the colorregions change as a result. In this case, region 2020 is red, whileregion 2022 is blue.

In FIG. 20B, a system is shown which can be used to quickly andaccurately assess the residual stress state of a deposited film. Wafer2030 comprises an array of IMod structures consisting of both single anddouble cantilevered membranes with varying lengths and widths. Thestructural membranes are fabricated from a material whose mechanical andresidual stress properties are well characterized. Many materials arepossible, subject to the limitations of the requisite reflectivity thatcan be quite low given that the IMods in this case are not to be usedfor display purposes. Good candidates would include materials incrystalline form (silicon, aluminum, germanium) which are or can be madecompatible from a fabrication standpoint, exhibit some degree ofreflectivity, and whose mechanical properties can or have beencharacterized to a high degree of accuracy. These “test structures” arefabricated and released so that they are freestanding. If the materialsare without stress, then the structures should exhibit no colorvariations. Should this not be the case, however, then the color statesor color maps may be recorded by use of a high resolution imaging device2034, which can obtain images of high magnification via optical system2032.

The imaging device is connected to a computer system 2036, upon whichresides hardware and capable of recording and processing the image data.The hardware could comprise readily available high speed processingboards to perform numerical calculations at high rates of speed. Thesoftware may consist of collection routines to collect color informationand calculate surface deformations. The core routine would use thedeformation data to determine the optimal combination of uniform stressand stress gradient across the thickness of the membrane, which iscapable of producing the overall shape.

One mode of use could be to generate a collection of “virgin” testwafers with detailed records of their non-deposited stress states, to beput away for later use. When the need arises to determine the residualstress of a deposited film, a test wafer is selected and the film isdeposited on top of it. The deposited film alters the geometry of thestructures and consequently their color maps. Using software resident onthe computer system, the color maps of the test wafer both before andafter may be compared and an accurate assessment of the residual stressin the deposited film made. The test structures may also be designed tobe actuated after deposition. Observation of their behavior duringactuation with the newly deposited films can provide even moreinformation about the residual stress states as well as the change inthe film properties over many actuation cycles.

This technique may also be used to determine the stress of films as theyare being deposited. With appropriate modification of the depositionsystem, an optical path may be created allowing the imaging system toview the structures and track the change of their color maps in realtime. This would facilitate real-time feedback systems for controllingdeposition parameters in an attempt to control residual stress in thismanner. The software and hardware may “interrogate” the test wafer on aperiodic basis and allow the deposition tool operator to alterconditions as the film grows. Overall this system is superior to othertechniques for measuring residual stress, which either rely onelectromechanical actuation alone, or utilize expensive and complexinterferometric systems to measure the deformation of fabricatedstructures. The former suffers from a need to provide drive electronicsto a large array of devices, and inaccuracies in measuring displacementelectronically. The latter is subject to the optical properties of thefilms under observation, and the complexity of the required externaloptics and hardware.

Discontinuous Films

Another class of materials with interesting properties are films whosestructure is not homogeneous. These films can occur in several forms andwe shall refer to them collectively as discontinuous films. FIG. 21Aillustrates one form of the discontinuous film. Substrate 2100 could bea metal, dielectric, or semiconductor, which has had contours 2104,2106, and 2108 etched into its surface. The contours, comprisingindividual structural profiles which should have a height 2110 that issome fraction of the wavelength of light of interest, are etched usingphotolithographic and chemical etching techniques to achieve profileswhich are similar to those illustrated by, 2104 (triangular), 2106,(cylindrical) and 2108 (klopfenstein taper). The effective diameter ofthe base 2102 of any of the individual profiles is also on the order ofthe height of the pattern. While each contour is slightly different,they all share in common the property that as one traverses from theincident into the substrate, the effective index of refraction goesgradually from that of the incident medium, to that of the filmsubstrate 2100 itself. Structures of this type act as superiorantireflection coatings, compared to those made from combinations ofthin films, because they do not suffer as much from angulardependencies. Thus, they remain highly antireflective from a broaderrange of incident angles.

FIG. 21B reveals a coating 2120 that has been deposited on substrate2122 and could also be of a metal, dielectric, or semiconductor. Thefilm, in this case, is still in the early stages of formation, somewherebelow 1000 angstroms in thickness. During most deposition processes,films undergo a gradual nucleation process, forming material localitiesthat grow larger and larger until they begin to join together and, atsome point, form a continuous film. 2124 shows a top view of this film.The optical properties of films in the early stage differ from that ofthe continuous film. For metals, the film tends to exhibit higher lossesthan its continuous equivalent.

FIG. 21C illustrates a third form of discontinuous film. In this case,film 2130 has been deposited on substrate 2132 to a thickness, at leasta thousand angstroms, such that it is considered continuous. A patternof “subwavelength” (i.e. a diameter smaller than the wavelength ofinterest) holes 2134 is produced in the material using techniques whichare similar to the self-assembly approach described earlier. In thiscase, the polymer can act as a mask for transferring the etch patterninto the underlying material, and the holes etched using reactive ionetch techniques. Because the material is continuous, but perforated, itdoes not act like the early stage film of FIG. 21B. Instead, its opticalproperties differ from the un-etched film in that incident radiationexperiences lower losses and may exhibit transmission peaks based onsurface plasmons. Additionally, the geometry of the holes as well as theangle of incidence and refractive index of the incident medium may bemanipulated to control the spectral characteristics of the light that istransmitted. 2136 shows a top view of this film. Films such as these aredescribed in the paper “Control of optical transmission through metalsperforated with subwavelength hole arrays” by Tae Jin Kim. While theyare regular in structure, they differ from PBGs.

All three of these types of discontinuous films are candidates forinclusion into an IMod structure. That is to say they could act as oneor more of the material films in the static and/or movable portions ofan IMod structure. All three exhibit unique optical properties which canbe manipulated in ways that rely primarily on the structure and geometryof the individual film instead of a combination of films with varyingthickness. They can be used in conjunction with other electronic,optical, and mechanical elements of an IMod that they could comprise. Invery simple cases, the optical properties of each of these films may bechanged by bringing them into direct contact or close proximity to otherfilms via surface conduction or optical interference. This can occur bydirectly altering the conductivity of the film, and/or by altering theeffective refractive index of its surrounding medium. Thus more complexoptical responses in an individual IMod may be obtained with simplerstructures that have less complex fabrication processes.

Other embodiments are within the scope of the following claims:

What is claimed is:
 1. An interferometric modulator comprising: astructure associated with actuation of the modulator; and aninterferometric cavity having walls, the structure being obscured by atleast one of the walls of the interferometric cavity.
 2. Aninterferometric modulator comprising: a body having a cavity defined bya front wall, a rear wall, and two sidewalls; a displaceable supportmember located within the cavity; a first mirror supported by thedisplaceable support member; a second mirror located within the cavityadjacent the front wall; and an electrode located within the cavityadjacent the rear wall, wherein an optical response of theinterferometric modulator is determined by a relative spacing betweenthe first and second mirrors, and wherein the electrode, when actuated,causes electrostatic displacement of the support member resulting in achange in the relative spacing between the first and second mirrors. 3.The interferometric modulator of claim 2, wherein the first mirrorcomprises a reflective layer deposited directly on the displaceablesupport member.
 4. The interferometric modulator of claim 2, wherein thefirst mirror comprises a reflective layer spaced from, and substantiallyparallel to, the displaceable support member and supported thereon by aspacer member.
 5. The interferometric modulator of claim 4, wherein thereflective layer obscures the displaceable support member, during allstages of operation of the interferometric modulator, when viewed froman end remote from the electrode.
 6. An interferometric modulatorcomprising: an optical component including first and second mirrorsmovably mounted relative to each other to allow a relative spacingbetween the mirrors to be adjusted to change an optical response of theoptical component; and an actuation mechanism including an electrode tocause electrostatic displacement of the first mirror relative to thesecond mirror to adjust the relative spacing between the mirrors.
 7. Theinterferometric modulator of claim 6, wherein the optical response isdetermined only by the first and second mirrors which cooperate tointerferometrically modulate incident light.
 8. The interferometricmodulator of claim 6, further comprising a displaceable support memberto support the first mirror.
 9. The interferometric modulator of claim8, wherein the first mirror comprises a reflective layer depositeddirectly on the displaceable support member.
 10. The interferometricmodulator of claim 9, wherein the first mirror comprises a reflectivelayer, spaced from, and substantially parallel to, the displaceablesupport member and supported thereon by a spacer member.
 11. Theinterferometric modulator of claim 10, wherein the displaceable supportmember is completely hidden by the reflective layer, during all stagesof operation of the interferometric modulator, when viewed from an endremote from the electrode.
 12. An interferometric modulator comprising:a substrate; and a stacked structure formed on the substrate, thestacked structure comprising an operatively lower electrode layer, amiddle first mirror layer normally spaced from the lower electrodelayer, and an operatively upper second middle layer normally spaced fromthe first mirror layer, wherein a spacing between the first and secondmirror layers is such that the first and second mirror layers cooperateto interferometrically modulate light incident upon the stackedstructure, and wherein actuation of the electrode layer causeselectrostatic displacement of the first mirror layer to change thespacing between the first and second mirror layers.
 13. Theinterferometric modulator of claim 12, wherein the stacked structurefurther comprises a movable support member to support the first mirrorlayer.
 14. The interferometric modulator of claim 13, wherein the firstmirror layer is deposited directly on the movable support member. 15.The interferometric modulator of claim 13, wherein the first mirrorlayer is mounted on a spacer member supported by the movable supportmember so as to be spaced from and substantially parallel to the movablesupport member.
 16. The interferometric modulator of claim 15, whereinthe first mirror layer completely obscures the movable support member,during all stages of operation of the interferometric modulator, whenviewed from an end remote from the electrode layer.